It is recognized in the stereophonic art that more noise is intrinsically present than in monaural systems. Monaural phonograph records, for example, require groove modulation in a single plane and monaural phonograph cartridges may be constructed so as to attenuate output from stylus vibrations in other planes. Thus, monaural cartridges may reduce or eliminate noise from record surface flaws or other vibrations which induce stylus motion in planes other than that of the intended groove modulation. Additionally, hum cancelling phasing of coils may be effected.
Single-groove stereophonic, discs, however, are customarily modulated simultaneously in two perpendicular directions, usually 45.degree. to the record surface. This results in lateral modulation representing the sum of the two stereophonic signals and vertical groove modulation representing the difference between the two signals. Stereophonic cartridges must, therefore, function in more than one plane and will respond to and produce outputs in the form of noise when vertical stylus motion is induced by record surface flaws, foreign material in record grooves, and vibrations from the playback mechanism. Such noise will appear in both channels with a phase difference between the channels of 180.degree..
Stereophonic FM broadcast signals are demodulated at approximately 20 db higher noise levels than equivalent monaural broadcasts. The intrinic characteristics of the multiplexing and subsequent demultiplexing process are such that the bulk of this additional noise appears in both channels with a phase difference between the channels of 180.degree.. Most of this noise is moderately broad-band ranging from approximately 1500 hz to 5000 hz and is audible as background hiss inversely proportional to the strength and proximity of the FM broadcast source.
In addition to predominately out-of-phase stereo noise sources, others are normally present which may be random in nature. Such noise may derive from passive components such as resistors, active components such as transistors, magnetic tape discontinuities, etc.
In consideration of the phase characteristics of the noise in stereo channels, it is apparent that elimination of the difference signal component, as measured between the channels, will reduce the noise content of the channels. Accordingly, some prior art FM receivers include provision for manually switching an impedance so as to partially intermix the stereo channels and cancel some noise at the expense of stereo separation.
Similarly, in prior art FM radio systems, reception may be switched from stereophonic to monaural reproduction automatically whenever the received signal strength falls below a threshold reception level of the 19 khz pilot subcarrier. In these systems, there is a sharp transition between stereo and monaural performance and available stereo program material is lost completely to reduce noise. These systems do nothing to reduce noise at signal strengths sufficient for stereophonic demodulation, but insufficient for full noise quieting.
Many prior art general application stereo noise suppressors maintain maximum stereo channel separation at all amplitudes and employ static and/or dynamic frequency responsive filters in each channel. Such devices are complex and expensive for applications requiring little degredation of original program material. Other prior art noise suppression systems employ combinations of expansion and compression and require, for effective results, that the dynamic range of the source material be altered prior to transmission or recording with subsequent restoration of the original dynamic range by additional equipment at the point of playback or reception. These systems are complex and require critical adjustments to preclude distortion of the original program material.